Polluted groundwater is the most significant route of exposure to toxic chemicals for many human populations who rely on wells for drinking water and other domestic use. Groundwater aquifers are extremely vulnerable to contamination due to migration of pollutants from Superfund and other hazardous waste sites. Unfortunately, it is very difficult to predict the potential for contamination of a groundwater aquifer and the rate of movement of pollutants from hazardous waste sites to the water table. The unsaturated subsurface (vadose) environment is extremely heterogeneous and its physical, chemical, and biological processes have not received the attention that such processes have in saturated, groundwater aquifers. The uncertainties in scaling up local processes in the vadose zone to the system scale impedes the development of the predictive models that are essential for describing subsurface transport and assessing the potential exposure of human populations. Volatile organic chemicals (VOCs), such as trichloroethylene, toluene, and naphthalene, are ubiquitous and common contaminants of groundwater. All three chemicals are E.P.A. priority pollutants and have the potential to be transformed in the environment into undesirable metabolites that pose a health risk to human populations. Studies will be conducted to investigate the fundamental processes-- including diffusive and convective transport, sorption/desorption, and biodegradation--affecting the fate and transport of VOCs in soil and geological sediments. Rates of biodegradation will be related to population size and the enzymatic and genetic properties of indigenous microbial communities in contaminated and uncontaminated vadose zone samples. The potential for formation of toxic metabolites of the VOCs during their biodegradation will be determined under various conditions. Laboratory-scale, column studies will be conducted using soil and geological sediments to determine the effect of multiple environmental processes on VOC concentrations and the potential for movement of VOCs to the groundwater and atmosphere. Relationships between VOC transport processes and field-scale geological attributes of the vadose zone such as depositional facies, grain size, organic carbon, sorting and mineralogy will be determined. Quantitative descriptions of these relationships will be incorporated into deterministic and stochastic models of transport in the vadose zone. These models will be run with data from a contaminated site at Lawrence Livermore National Laboratory to determine the sensitivity of migration rates and vapor and solution phase concentrations of VOCs and their metabolites to spatial variability in transport characteristics.